Abstract

A Neutral-Point-Clamped (NPC) three-level
inverter with small DC-link capacitors is presented in this
paper. This inverter requires zero average neutral-point
current for stable neutral-point potential. A simple carrier
based modulation strategy is proposed for achieving zero
average neutral-point current. A new simplified model of the
neutral-point voltage dynamics is derived for the proposed
modulation strategy. This model shows that the neutral-point
current is proportional to the power drawn from the converter
and it enables the use of well established classical control
theory for neutral-point voltage controller design. A PI
controller is used for neutral-point voltage balance. The
implementation of the proposed modulation strategy and
controller is simple. It does not require any information about
the output phase currents. The controller is implemented in a
7.5 kW induction machine based drive with only 14 μF DC-link
capacitors. A fast and stable performance of the neutral-point
voltage controller is achieved and verified by experiments.